Highly efficient blue phosphorescent organic light-emitting diodes (PhOLEDs) with a multiple quantum well (MQW) structure were investigated. A peak external quantum efficiency (EQE) of 20.31%, current efficiency of 40.31 cd/A and power efficiency of 30.14 lm/W were achieved in the optimized device with two quantum wells (QWs). The obtained efficiencies are much higher than those of the control devices without QWs. More importantly, the MQW devices exhibit low efficiency roll-off. At a high luminance of 5000 cd/m(2), the EQE still keeps at a high value of 18.86% in the optimized MQW device, and the efficiency roll-off is only 7.14%, which is lower than that of 30.78% in the control device (reduced from 16.05% to 11.11%). Meanwhile, the maximum power efficiency of the optimized MQW device was also exhibited more than 54.80% improvement compared to the control device. The high efficiency and low efficiency roll-off are attributed to the effective confinement of charge carriers and excitons by the state-of-the-art MQWs.